Classic fun.
Anyway, the online version doesn't have the reverse button, which makes it more playable but not as "classic" a representation... I'm sure you can download another port of it off many sites, search "defender" or "stargate".
Gameplay: Fly around a small landscape. Save people from aliens that fly down to abduct them and eat them at the top of the screen. Fight off thousands of other aliens. The enemies are quite varied, with "pods" that look innocent enough but will explode if you shoot them into 5 or 6 little "swarmers"... "firebombers" that look like bladed death frisbees and will launch firebombs (what else?) at you... "phreds" that look like square pac-men and will launch "mini-reds" at you... and of course, "baiters", returning from the original game, which shoot you, outrun you, are a small target, and are otherwise a nuisance.
For some reason this game is really addictive, and my friends and I used to spend hours seeing who could get the highest score (on the internet version)... I don't think we ever broke 40000, or made it past wave 4, but at the end we were consistently making it to wave 3, and scoring over 25000 points.
Graphics: 2d, but very shiny and shimmery. You kind of have to see them.
Sound: Great sound effects. I wouldn't mind if a newer shooter had the same "blaster" sounds.
Value: Free... minus the cost of your internet connection.
Reviewer's Tilt: Very high. This is my favorite online game. Try it!
One last note: Get all the pods in your screen (watch the "pod intersection" clock) and hit the smart bomb for thousands of points!